Electrodeless high intensity discharge (HID) lamps have been described extensively in the prior art. In general, electrodeless HID lamps include an electrodeless lamp capsule containing a volatizible fill material. The lamp capsule is mounted in a fixture which is designed for coupling high frequency energy to the lamp capsule. The high frequency energy produces a light emitting plasma discharge within the lamp capsule. Recent advances in the application of microwave power to lamp capsules operating in the tens of watts range are disclosed in U.S. Pat. No. 5,070,277 issued Dec. 3, 1991 to Lapatovich; U.S. Pat. No. 5,130,612 issued Jul. 4, 1992 to Lapatovich et al; U.S. Pat. No. 5,241,246 issued Aug. 31, 1993 to Lapatovich et al and U.S. Pat. No. 5,144,206 issued Sep. 1, 1992 to Butler et al. As a result, compact electrodeless HID lamps and associated applicators have become practical.
These patents describe small cylindrical lamp capsules wherein high frequency energy is coupled to opposite ends of the lamp with a 180.degree. phase shift. The applied electric field is colinear with the arc tube axis and produces a substantially linear discharge within the lamp capsule. The disclosed lamps provide highly satisfactory performance. However, the high frequency power (and electric field) is concentrated at the ends of the lamp capsule and produces hot spots and potential failures in the vicinity of the field applicators.
Lamps with different geometries have been disclosed in U.S. Pat. No. 4,803,404 issued Feb. 7, 1989 to Anderson; U.S. Pat. No. 4,972,120 issued Nov. 20, 1990 to Witting and U.S. Pat. No. 4,887,008 issued Dec. 12, 1989 to Wood. These patents disclose relatively high power devices with relatively large dimensions. In addition, power is coupled to the disclosed lamps through magnetic induction rather than capacitive coupling.
A microwave lamp energized by a rotating field of constant ellipticity is disclosed in U.S. Pat. No. 5,227,698 issued Jul. 13, 1993 to Simpson et al. An electrodeless lamp is mounted within a resonant cavity which includes a solid metallic portion and a mesh portion. Microwave energy is coupled through slots in the resonant cavity to produce a rotating field. Waveguides are used to couple microwave energy from a source to the slots in the resonant cavity. The disclosed lamp is relatively large and expensive. Coupling microwave energy to a lamp located in a resonant cavity also requires placing the lamp in an optically opaque metal structure. It is inefficient to couple light out of a structure of this type.
An electrodeless lamp wherein the lamp capsule is rotated in a stream of compressed air within a resonant cavity to facilitate cooling is disclosed in U.S. Pat. No. 4,975,625 issued Dec. 4, 1990 to Lynch et al. This configuration has the disadvantage of requiring a motor for rotating the lamp capsule, and forced air cooling. Filtering of the cooling air is also required to prevent abrasion of the lamp by dust particles and premature lamp failure. Additional electrodeless lamps are disclosed in U.S. Pat. No. 4,504,768 issued Mar. 12, 1985 to Ury et al; U.S. Pat. No. 4,749,915 issued Jun. 7, 1988 to Lynch et al and U.S. Pat. No. 4,954,755 issued Sep. 4, 1990 to Lynch et al.